Double-acting shock absorber with volume compensation for the stroke of the rod

ABSTRACT

Double-acting shock absorber, comprising a piston-cylinder shock absorber which is known from the prior art. During the reciprocating movement of the piston, which is provided on one side with a piston rod which extends to outside the cylinder, an interplay of forces of different magnitudes is generated in the different directions of movement. This is caused by the fact that the surface area on which the liquid situated in the shock absorber acts is smaller on that side of the piston rod which has the surface of the piston rod than on the other side of the piston. According to the invention, these forces are made substantially equal by connecting said other side of the piston to a compensation chamber of variable volume which essentially corresponds to the instantaneous volume displaced by the piston rod in the cylinder.

The present invention relates to a double-acting shock absorberaccording to the preamble of claim 1.

Shock absorbers of this kind are generally known in the prior art andare used for damping all kinds of vibrating, oscillating and impactmovements. The interplay of forces during the inwards and outwardsstrokes is not symmetrical in such cases, since the first chambercontains the piston rod, with the result that the surface area of liquidwhich acts on that side of the piston is less than the effective surfacearea of the second chamber. In order to avoid this drawback, it has beenproposed in the prior art to lengthen the piston rod, so that it alwaysextends completely through the cylinder. This has the drawback that itis necessary to provide further sealing means at the location of thatpart of the piston rod which projects out of the cylinder but has nofurther function. Moreover, with a design of this nature it is no longerpossible to arrange fastening means, such as eyelets, on the axis of thestroke movement. Fastening means of this kind then have to be arrangedon the side. Consequently, on the one hand more space is required, andon the other hand the transmission of forces is less effective, due tothe fact that a moment is generated. For this reason, in manyapplications the drawback of the unequal effective surface area onopposite sides of the piston is accepted, or else the fastening means isdisplaced so far into the axis that the emerging piston rod meets nolonger any obstruction.

However, in the art shock absorbers are increasingly being used fordamping movements of very low amplitude, i.e. movements of an amplitudewhich is significantly less than 1 cm, being in the order of magnitudeof millimetres. A very rigid shock absorber is needed in such cases ifoptimum shock absorption is to be achieved. Since the shock absorber isdimensioned on the basis of the minimum damping which it has to carryout, i.e. that with the smallest piston surface area, i.e. where thepiston rod is situated, this situation means that the shock absorber hasto be oversized, for the shock absorber has to be calculated on thebasis of the highest load occurring, which is generated by that surfaceof the piston which lies on the opposite side. This design of increasedweight runs counter to the desire to provide more lightweightstructures, and is no longer acceptable in certain applications.

From U.S. Pat. No. 2,057,893 A a shock absorber is known in which somecompensation is realized by providing a third chamber having a changeingvolume. The volume of this third chamber increases as the piston rodmoves further into the first chamber and the third chamber is incommunication with the second chamber. Because this is a gas filledshock absorber a further compensation is obtained by the gas inside theabsorber.

The object of the present invention is to avoid the drawback describedabove and to provide a double-acting shock absorber which saves space,can be of optimum design for both the inwards and outwards stroke withregard to the strength thereof, and in which moreover it is possible toattach the fastening means at any desired location. Furthermore fullcompensation of the volume of the piston rod is aimed.

This object is achieved in a double-acting shock absorber as describedabove with the characterising features of claim 1. The presence of athird chamber provides compensation for the presence of the piston rod.This means that in the event of equal loading during the inwards andoutwards stroke, equal forces are generated inside the shock absorber.This is important in particular for shock absorbers which execute ashort stroke and therefore have to be particularly (oil)rigid.

The volume of the third chamber, and more particularly the changethereof, is always essentially equal to the change in volume in thefirst chamber caused by the inward or outward movement of the pistonrod. This means that the total volume of the first and second chambersremains constant despite the inward or outward movement of the pistonrod, owing to the change in volume of the third chamber. Volumecompensation of this nature can be achieved particularly easily if thethird chamber is also delimited by the first and second parts, i.e. its“length” also changes when the first and second parts move with respectto one another. By then selecting the surface area to be equal to thesurface area of the piston rod, it is possible to provide completecompensation for the volume of the piston rod.

According to an advantageous embodiment of the invention, the cylinderis externally provided with a plunger, and the second part is providedwith a cylindrical circumferential part, which is designed to interactwith the plunger and is provided, in the region of a free end, withfurther sealing means, in order to delimit the third chambertherebetween.

In such a case, the exterior of the cylinder, i.e. of the first part,can function as a plunger rod for the plunger, with the circumferentialpart of the second part being provided with suitable sealing means. Theabove-described plunger and the conventional sealing means for theconventional piston rod can form a unit. Moreover,expansion-compensation means for the shock-absorber liquid may bearranged therein. This is because this liquid will expand slightly whenheated, a fact which is noticeable particularly in the case ofrelatively rigid shock absorbers and has to be compensated for.

The actual shock absorber itself, i.e. the piston-cylinder assembly, maybe designed in any manner which is known from the prior art. Thisrelates in particular to the design of the passages for the oil, whichmay be provided with valves which are optionally externally actuable.Electrical adjustment is possible, and it is also possible to provide avolume of gas in order to assist the functioning of the liquid.

It is stated above that the shock absorber according to the inventioncan be used on any structure according to the prior art where it isnecessary to provide damping between two moving parts. In particular,this shock absorber can be used as a vibration or rotation absorber forwheel sets, and more particularly wheel sets which are used in railways.

The invention will be explained in more detail below with reference toan exemplary embodiment which is illustrated in the drawing, in which:

FIG. 1 shows a cross-section through the shock absorber according to theinvention;

FIG. 2 shows the shock absorber in accordance with FIG. 1 with thepiston moved further inwards;

FIG. 3 shows detail III from FIG. 1 on an enlarged scale; and

FIG. 4 shows detail IV from FIG. 1 on an enlarged scale.

In FIG. 1, 1 denotes the shock absorber according to the invention, in afirst position. This shock absorber comprises a first part 2 and asecond part 3, which can be moved in a reciprocating manner with respectto one another. With the aid of fastening eyelets 4, the ends of theshock absorber parts can be fastened to a vehicle. As a non-limitingexample, it is possible to mention attachment of a bogie to the chassisor frame of a railway vehicle, in order to absorb vibrational movementsof wheel sets as far as possible.

The first part 2 comprises a cylinder 5 and is provided at the end witha sealing stop 6, more details of which can be seen in FIG. 3. At theend of cylinder 5 there is arranged an opening 7 which adjoins bores 8.The second part 2 is provided with a piston 10 which is arranged incylinder 5. This piston 10 is provided with spring-loaded valves 11,which shut off ducts 13. Moreover, piston 10 is provided with a seal 12.Details of this piston are not important to the present application andmay comprise any structure which is known from the prior art. Via pistonrod 14, piston 10 is connected to eyelet 4. It can be seen from FIG. 3that the sealing stop 6 is provided with a bearing bush 15 over whichpiston rod 14 runs. A first, high-pressure seal is denoted by 16, whilethe second, low-pressure seal is denoted by 17. Moreover, there is acompensation device in sealing stop 6 which compensates for expansionand/or contraction of the volume of liquid as a result of a rise intemperature. This compensation device comprises a compensation chamber19, which is delimited by-sealing stop 6 and retaining ring 18. Thiscompensation chamber is connected to a duct 22 which is closed off by apiston 21 which is spring-loaded by means of spring 20. The strength ofspring 20 is selected in such a manner that in normal operation piston21 will never move far enough for liquid to be able to pass into chamber19. Moreover, there is a vacuum system (not shown in more detail), withthe result that not only can liquid flow out of first chamber 25 in theevent of high pressure in the compensation chamber 19, but also, in theevent of reduced pressure, i.e. cooling, liquid can flow back. It isclear that chamber 19 will be partly filled with a pressurized gas. Duct23 is present in order to return leaked liquid which is situated betweenhigh-pressure sealing ring 16 and low-pressure sealing ring 17 to thesystem, in order in this way to prevent leakage.

A cylindrical circumferential part 24 also forms part of the second part3. This is dimensioned in such a manner that the outer part of sealingstop 6, which acts as a plunger, is held in a sealing manner. For thispurpose, this outer part of sealing stop 6, as can be seen from FIG. 3,is provided with a bearing surface 35, a high-pressure seal 36 and alow-pressure seal 37. In this case too, a return duct which opens outinto pressure-compensation chamber 19 is arranged between thehigh-pressure seal and the low-pressure seal. The free end of thecylindrical circumferential part 24 is provided with a sealing bush 28,for example by means of a screw connection (FIG. 4). This sealing bushis also provided with a bearing surface 29, a high-pressure sealing ring30, a low-pressure sealing ring 31 and a return duct 32 which extendsbetween the two sealing rings. This return duct opens out into aspring-loaded valve 33. Sealing bush 28, and more particularly the partswhich have just been described, are designed to run over the outersurface of cylinder 5. The third chamber 27 is delimited in the partbetween sealing bush 28 and sealing stop 6. Piston 10 delimits, on oneside, the first chamber 25 which has already been mentioned above and,on the other side, the chamber 26. Via opening 7, bore'8 provides aconnection between second chamber 26 and third chamber 27.

The surface area of the third chamber 27, i.e. the surface area of theannular part which extends perpendicular to the axis 9, is essentiallyequal to the cross-sectional surface area of the piston rod 14.

On comparing FIGS. 1 and 2, it will be seen that as the piston 10 movesfurther inwards into cylinder 5, the volume of second chamber 26 on theone hand decreases owing to the inward movement of this piston, but onthe other hand apparently increases, owing to the fact that-some of thefluid can flow out into the third chamber 27, which becomes larger. Theextent to which this third chamber 27 becomes larger correspondsprecisely to the increased volume of the piston rod 14 which is situatedin the first chamber 25.

In this way, the forces generated in the liquid in the event of tensileor compressive movements between the fastening eyelets 4 can be madeequal, due to the fact that uniform loading of the shock absorber isprovided, so that the latter can function optimally. This isparticularly, although not exclusively, important for shock absorberswhich have a relatively short stroke of, for example, 1-2 mm.

Although the invention is described above with reference to a preferredembodiment, it will be understood that numerous modifications can bemade to this embodiment. For the sake of simplicity, the controlarrangement for the shock absorption is positioned in the piston. Itwill be understood that it may also be arranged at any other location inthe shock absorber and moreover can be designed so that it can beadjusted in any desired manner (electrically). Moreover, the thirdchamber can be realized in some other manner, while the remainingcomponents can be modified in ways which are generally known in theprior art. Moreover, it is possible to eliminate any problems which mayarise from the excessively fixed design of the structure by employing afloating, hydraulically balanced piston at the location of theattachment of the piston rod 14 to the second part 3. All variations ofthis nature are considered to lie within the scope of the appendedclaims.

What is claimed is:
 1. Double-acting shock absorber, comprising: a firstpart, which is provided with fastening means for fastening to a firstcomponent of a vehicle and comprises a cylinder, a second part, which isprovided with fastening means for fastening to a second component ofsaid vehicle and comprises a piston, which is designed to move insidesaid cylinder and thus delimits, on opposite sides of the piston, afirst chamber and a second chamber, a piston rod, which is situated insaid first chamber, being arranged between said fastening means of saidsecond part and said piston, a liquid which is arranged in said firstand second chamber for transmitting forces from said piston to saidcylinder, or vice versa, flow ducts, which allow movement of said liquidfrom the first chamber to the second chamber, or vice versa, a thirdchamber delimited between said first and second parts, the volume ofwhich third chamber increasing as the piston rod moves further into thefirst chamber, and which third chamber is in communication with thesecond chamber, the volume of the third chamber is defined by thesurface area which extends perpendicular to the stroke movement of thepiston and by the path covered by the piston rod, said surface areabeing substantially equal to the surface area of the piston rodperpendicular to its direction of movement.
 2. Shock absorber accordingto claim 1, in which the outside of the cylinder is provided with aplunger, and the second part is provided with a cylindricalcircumferential part, which is designed to interact with said plungerand is provided, in the region of a free end, with sealing means, inorder to delimit said third chamber therebetween.
 3. Shock absorberaccording to claim 2, in which the outside of the cylinder is designedas a plunger rod in order to act on said sealing means.
 4. Shockabsorber according to claim 1, in which said communication between thethird chamber and the second chamber comprises a bore which extends inthe wall of the cylinder of the first part.
 5. Shock absorber accordingto claim 2, in which said plunger comprises an integrated unit, in whichthere are arranged expansion-compensation means for liquid.
 6. Shockabsorber according to claim 1, in which said piston is provided withflow valve controlled flow ducts.
 7. Shock absorber according to claim1, in which the passage through said flow ducts can be externallyinfluenced by electrical means.
 8. Shock absorber according to claim 1,with an effective stroke of less than 1 cm.
 9. Shock absorber in arailway vehicle, according to claim 1, to absorb vibrating movementsbetween a bogie and a chassis.